drm/amd/pm: only check sriov vf flag once when creating hwmon sysfs

The current code checks sriov vf flag multiple times when creating
hwmon sysfs. So fix it.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>